Changing default mesh size
This won't be an instantly workable answer, because this is an issue that I'm having trouble with, too.

But, it seems like you could maybe unregister the operators, then reregister them with your modifications. Checking out the Python API, I found: https://docs.blender.org/api/current/bpy.ops.mesh.html  

eg: 

unregister()

    bpy.ops.mesh.primitive_cube_add(size=2.0, calc_uvs=True, enter_editmode=False, align='WORLD', location=(0.0, 0.0, 0.0), rotation=(0.0, 0.0, 0.0))

register()

    bpy.ops.mesh.primitive_cube_add(size=20.0, calc_uvs=True, enter_editmode=False, align='WORLD', location=(0.0, 0.0, 0.0), rotation=(0.0, 0.0, 0.0))


or something like that in an __init__ file?

I hope you get an answer, as I'd like to do the same for meshes, but also clip distance for cameras!